Abstract. The chemical composition of aerosol particles (Dp ≤ 1.5 μm)
was measured over the southeast Pacific Ocean during the VAMOS (Variability of the American Monsoon
Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-Rex) between 16 October and 15 November 2008 using the US Department of Energy (DOE) G-1
aircraft. The objective of these flights was to gain an understanding of the
sources and evolution of these aerosols, and of how they interact with the
marine stratus cloud layer that prevails in this region of the globe. Our
measurements showed that the marine boundary layer (MBL) aerosol mass was
dominated by non-sea-salt SO42−, followed by Na+, Cl−,
Org (total organics), NH4+, and NO3−, in decreasing order of importance;
CH3SO3− (MSA), Ca2+, and K+ rarely exceeded their
limits of detection. Aerosols were strongly acidic with a NH4+ to
SO42− equivalents ratio typically < 0.3. Sea-salt aerosol
(SSA) particles, represented by NaCl, exhibited Cl− deficits caused by
both HNO3 and H2SO4, but for the most part were externally
mixed with particles, mainly SO42−. SSA contributed only
a small fraction of the total accumulation mode particle number
concentration. It was inferred that all aerosol species (except SSA) were of
predominantly continental origin because of their strong land-to-sea
concentration gradient. Comparison of relative changes in median values
suggests that (1) an oceanic source of NH3 is present between 72° W and
76° W, (2) additional organic aerosols from biomass burns or biogenic
precursors were emitted from coastal regions south of 31° S, with possible
cloud processing, and (3) free tropospheric (FT) contributions to MBL gas and aerosol
concentrations were negligible. The very low levels of
CH3SO3− observed as well as the correlation between
SO42− and NO3− (which is thought primarily
anthropogenic) suggest a limited contribution of DMS to SO42−
aerosol production during VOCALS.